How to initialize List<String> object in Java?

Question

I can not initialize a List as in the following code:

List<String> supplierNames = new List<String>();
supplierNames.add("sup1");
supplierNames.add("sup2");
supplierNames.add("sup3");
System.out.println(supplierNames.get(1));

I face the following error:

Cannot instantiate the type List<String>

How can I instantiate List<String>?

Answer 1

If you check the API for List you'll notice it says:

Interface List<E>

Being an interface means it cannot be instantiated (no new List() is possible).

If you check that link, you'll find some classes that implement List:

All Known Implementing Classes:

AbstractList, AbstractSequentialList, ArrayList, AttributeList, CopyOnWriteArrayList, LinkedList, RoleList, RoleUnresolvedList, Stack, Vector

Some of those can be instantiated (the ones that are not defined as abstract class). Use their links to know more about them, I.E: to know which fits better your needs.

The 3 most commonly used ones probably are:

 List<String> supplierNames1 = new ArrayList<String>();
 List<String> supplierNames2 = new LinkedList<String>();
 List<String> supplierNames3 = new Vector<String>();

---

Bonus:
You can also instantiate it with values, in an easier way, using the Arrays class, as follows:

List<String> supplierNames = Arrays.asList("sup1", "sup2", "sup3");
System.out.println(supplierNames.get(1));

But note you are not allowed to add more elements to that list, as it's fixed-size.

Answer 2

Can't instantiate an interface but there are few implementations:

JDK2

List<String> list = Arrays.asList("one", "two", "three");

JDK7

//diamond operator
List<String> list = new ArrayList<>();
list.add("one");
list.add("two");
list.add("three");

JDK8

List<String> list = Stream.of("one", "two", "three").collect(Collectors.toList());

JDK9

// creates immutable lists, so you can't modify such list 
List<String> immutableList = List.of("one", "two", "three");

// if we want mutable list we can copy content of immutable list 
// to mutable one for instance via copy-constructor (which creates shallow copy)
List<String> mutableList = new ArrayList<>(List.of("one", "two", "three"));

Plus there are lots of other ways supplied by other libraries like Guava.

List<String> list = Lists.newArrayList("one", "two", "three");

Answer 3

List is an Interface, you cannot instantiate an Interface, because interface is a convention, what methods should have your classes. In order to instantiate, you need some realizations(implementations) of that interface. Try the below code with very popular implementations of List interface:

List<String> supplierNames = new ArrayList<String>(); 

or

List<String> supplierNames = new LinkedList<String>();